First direct measurements of N2 fixation during a Trichodesmium bloom in the eastern Arabian Sea

Abstract

We report the first direct estimates of N2 fixation rates measured during the spring, 2009 using the 15N2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N2 fixation rates vary from ∼0.1 to 34 mmol N m−2d−1 after correcting for the isotopic under-equilibrium with dissolved air in the samples. These higher N2 fixation rates are consistent with higher chlorophyll a and low δ15N of natural particulate organic nitrogen. Our estimates of N2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.